1. Field of the Invention
The present invention relates to voltage regulators and more specifically to multiphase direct current voltage regulators using voltage mode control.
2. Description of the Related Art
Computer systems in general and International Business Machines (IBM) compatible personal computer systems in particular have attained widespread use for providing computer power to many segments of today's modern society. A personal computer system can usually be defined as a desk top, floor standing, or portable microcomputer that includes a system unit having a system processor and associated volatile and non-volatile memory, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage device, and an optional printer. One of the distinguishing characteristics of these systems is the use of a system board to electrically connect these components together. These personal computer systems are information handling systems which are designed primarily to give independent computing power to a single user (or a relatively small group of users in the case of personal computers which serve as computer server systems) and are inexpensively priced for purchase by individuals or small businesses. A personal computer system may also include one or a plurality of I/O devices (i.e. peripheral devices) which are coupled to the system processor and which perform specialized functions. Examples of I/O devices include modems, sound and video devices, or specialized communication devices. Mass storage devices such as disk drive systems which include hard disks drives, CD-ROM drives or magneto-optical drives are also considered to be peripheral devices. Other types of computer systems include network servers which provide shared storage to a network of computer users.
One problem with computer systems as well as other electrical devices is how to provide power to the various components of the computer system within the voltage tolerance ranges required by those components. Power is usually supplied to a computer system from an alternating current (AC) source and converted to a direct current (DC) power by an AC to DC converter. DC power can also be supplied from a battery source as well. The DC output of a converter or battery source is then regulated by a DC regulator to provide DC power within specified tolerances.
One type of DC voltage regulator is a pulse width modulated "buck" regulator. Buck regulators typically include a switch in the form of a transistor in series with an unregulated DC power source. Connected to the other side of the switch is an inductor-capacitor (LC) circuit and diode. An oscillating circuit switches on and off the transistor to periodically provide current to the LC circuit from the unregulated DC power source. The LC circuit smoothes out the modulated current from the DC power source to provide an essentially ripple-free DC voltage. The longer that the switching transistor is switched on, the higher the DC voltage is at the output of the LC circuit. The regulator usually includes a feedback circuit to control the amount of time that the switching transistor is turned on and correspondingly, the amount of current provided to the transistor from the DC power source.
One problem with this type of DC regulator is that it consumes power in regulating the voltage and, consequently, generates heat which can significantly shorten the working life of the components in the computer system. A dual phase regulator is a more efficient regulator that includes two inductors in parallel with each inductor, on average, carrying half of the current provided to the LC circuit. Consequently, the power consumed, and therefore the heat produced, is significantly reduced. Another advantage of a multiphase regulator is that the heat generated by the switching can be spread out as opposed to being concentrated in a signal area as with a single phase system.
One problem with a dual phase regulator is that the "on-time" of one inductor needs to be nearly equal to the on-time of the other inductor, else one of the inductors will disproportionately carry a majority of the current load, and thus eliminating the benefits of a dual phase system. To remedy this problem, current mode control feedback is used to try to equalize the currents. However, a current mode control feedback consumes power and therefore produces heat. In addition, current mode control feedback requires current sensor resistors which can add to the cost of the circuit.
What is needed is an improved from of feedback for a multiphase DC regulator.